In some circuit applications for vertical double-diffusion MOS (VDMOS) transistors, the voltage between the gate and source electrodes may reach values very close to the breakdown voltage of the gate dielectric. This may cause degradation of the dielectric, or may even reach the breakdown value of the gate dielectric so that the transistor becomes unusable.
A known circuit which uses a VDMOS transistor is shown schematically in FIG. 1. It is a current generator comprising an npn bipolar transistor, indicated T1, and an n-channel VDMOS transistor, indicated T2, connected to the npn transistor in a "cascode" arrangement. The emitter of the npn transistor is connected to a supply terminal, indicated by the ground symbol. The drain terminal of the VDMOS transistor is connected, via a load RL, to a second supply terminal, indicated VDD. A predetermined voltage VR is applied to the base of the npn transistor and the gate terminal of the VDMOS transistor is connected to a reference-voltage generator represented by a Zener diode DZ in series with a constant-current generator G.
Between the collector of the npn transistor T1 and the ground terminal there is a parasitic diode D1 which represents the reverse-biased junction between the n-type collector region and the integrated circuit p-type substrate, which is connected to the ground terminal as is the emitter of the transistor. Between the drain and source terminals of the VDMOS transistor T2 there is a further parasitic diode D2 which represents the reverse-biased junction between the body region of the transistor, which is in contact with the source electrode, and the drain region of the transistor.
The circuit described above can supply to the load RL a current having a value which depends upon the supply voltage VDD, upon the voltage VR, and upon the reverse-conduction threshold of the Zener diode DZ. A particularly critical operating condition arises when the transistor T1 is cut off. In this situation, the generator current results solely from the leakage currents of the two diodes D1 and D2. A voltage which depends upon the supply voltage VDD and upon the equivalent resistances of the two reverse-biased diodes is established between the circuit node N, which is the connecting point between the collector of the transistor T1 and the source of the transistor T2, and ground.
These equivalent resistances may vary considerably from one example of the integrated circuit to another. This is so because of inevitable variations in manufacturing parameters and, in some cases, the potential of the node N, since a given supply voltage VDD may reach quite high values. Situations may therefore arise in which the voltage between the node N, that is, between the source terminal of the transistor T2 and the gate terminal of the transistor, approach or reach the breakdown voltage of the gate dielectric.
A known measure for attempting to prevent the dangerous situation described above is that of connecting between the source and gate terminals of the VDMOS transistor a voltage-limiting device. This may be, for example, a diode which has a predetermined reverse-conduction voltage and which becomes conductive at a voltage between the source and gate and lower than that considered dangerous to the VDMOS transistor. Naturally, this approach applies not only to the current generator described above but, more generally, to the protection of VDMOS transistors in all applications in which the voltage between the source and gate may reach dangerous values. However, the voltage limiter and the respective connections take up additional area on the semiconductor chip containing the integrated circuit.